1. Technical Field
This disclosure relates to an image forming apparatus, and more specifically to an image forming apparatus including a liquid ejection head for ejecting liquid droplets.
2. Description of the Related Art
Image forming apparatuses are used as printers, facsimile machines, copiers, plotters, or multi-functional devices having two or more of the foregoing capabilities. As one type of image forming apparatuses employing a liquid-ejection recording method, inkjet recording apparatuses are known that use a recording head for ejecting droplets of ink.
Several different types of liquid ejection heads are known as recording heads usable in such liquid-ejection-type image forming apparatuses. One example is a piezoelectric head that ejects droplets by deforming a diaphragm using, e.g., piezoelectric actuators. When the piezoelectric actuators deform the diaphragm, the volumes of chambers containing the liquid change, thus increasing the internal pressures of the chambers to eject droplets from the head. Another example is a thermal head that ejects droplets by increasing the internal pressures of chambers with, e.g., heaters disposed in the chambers. The heaters are heated by electric current to generate bubbles in the chambers. As a result, the internal pressures of the chambers increase, thus ejecting droplets from the head.
For such liquid-ejection type image forming apparatuses, there is demand for enhancing throughput, i.e., speed of image formation. One way to increase the throughput is to enhance the efficiency of liquid supply. For example, a tube supply method is proposed to supply ink from a large-volume ink cartridge (main tank) mounted in an image forming apparatus to a head tank (also referred to as a sub tank or buffer tank) mounted in an upper portion of the recording head through a tube.
Such a tube supply method can reduce the weight and size of a carriage unit mounting the recording (liquid ejection) head and the head tank, thus reducing the size of the image funning apparatus including a structural system and a driving system.
However, for example, an increase in the number of nozzles of the head, an increase in the flow amount of ink feeding associated with use of higher frequencies in driving the head, and an increase in the viscosity of ink to reduce drying time may be advanced to further enhance printing throughput. As a result, a pressure loss due to a fluid resistance of a tube against a flow of ink may cause an ink supply shortage. In particular, an image forming apparatus capable of recording images on large-size print media may have a long tube, thus causing a large pressure loss.
Hence, for example, JP-4572987-B1 (JP-2009-143244-A) proposes to provide a pressure-difference regulation valve at an upstream side from a recording head in an ink supply direction to supply ink to the recording head only when a negative pressure in the head tank is greater than a predetermined pressure value. Such a configuration allows pressurization of ink in a supply tube to cancel a pressure loss in the supply tube.
In a liquid-ejection-type image forming apparatus, a filter may be disposed at a recording head to filter ink to be supplied to the recording head, which may cause a failure in bubble discharge.
For example, air may intrude into an ink supply channel due to a variety of causes, such as introduction of air on installation and removal of an ink cartridge or permeability of components of the ink supply channel. Such air intruding into the head may cause failures, such as ejection failure. Here, for air intruding from an upstream side of the ink supply channel, the filter near the head prevents such air from intruding into the head. As a result, such air may accumulate at an upstream side of the ink supply channel from the filter in the ink supply direction. When such air contacts a surface of the filter, ink does not flow in a contact area of air with the filter. As a result, when a certain amount of air accumulates at the upstream side of the ink supply channel, such air needs to be discharged from the recording head.
Hence, for example, JP-4572987-B1 (JP-2009-143244-A) proposes a bubble discharge method with choke cleaning. In the method, with a supply channel closed, liquid is sucked from nozzles of the recording head. After drastically reducing the pressure of a filter unit, the supply channel is opened. As a result, a high-speed ink flow arises in a filter unit, thus passing and discharging bubbles through the filter.
To perform high speed printing with highly viscous ink, it is preferable to increase the area of the filter to reduce resistance against ink flow to prevent ink supply shortage. As described above, for the bubble discharge method with choke cleaning described in JP-4572987-B1 (JP-2009-143244-A), such an increased area of the filter may cause insufficient bubble discharge performance.